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of the wing in the Cessna Caravan. This technique was possible mainly due to the lightweight structure of the wingtip system, which has a total weight of only 15 kg for each wingtip. Table 4. Parameters of the EM equipments in Twin Otter and Cessna Caravan Geological Survey of Finland, Special Paper 39 The “Three-in-One” aerogeophysical concept of GTK in 2004 Some technical specifications are listed in Tables 4 and 5. More detailed description of the EM system is given by Poikonen et al. (1998). Frequencies Coil spacing Sensitivity Range Registration rate (Hz) (m) (samples/sec) Twin Otter 3 125 and 14 368 21.36 1 ppm 1-50 000 ppm 4 Cessna 3 005 and 14 368 16.96 1 ppm 1-50 000 ppm 4 Table 5. Properties of the EM system in Twin Otter and Cessna Caravan. Noise is determined in real flight situation and it includes the movement noise from the wings. Twin Otter Cessna Caravan Frequencies (Hz) 3 125 14 368 3 005 14 368 Magnetic moment (A/m 2 ) 115 55 50 18 Noise STD/In-phase (ppm) 55 63 13 69 Noise STD/Quadrature (ppm) 24 58 9 60 The Earth’s magnetic field is measured with cesium magnetometers. The sensitivity of the cesium magnetometer is 0.001 nT. This is a great improvement in accuracy in comparison to that of the proton magnetometers (0.5nT), which were in use until 1992. Another improvement has been in the gradient tolerance of the cesium magnetometers. Proton magnetometers have poor operational characteristics when measuring in a high magnetic gradient environment. This is critical, especially when flying at very low altitudes and close to strong magnetic anomaly sources. The Finnish experience was that surveys using proton magnetometers recorded data gaps in areas of high magnetic gradients. The influence and the disturbance of the aircraft on the magnetometer recordings were minimised in two main ways: adjusting the location and the manner of installation of the sensor and applying software corrections. The first step was to mount the magnetometer sensor as far as possible from the aircraft itself. The mounting has to be as rigid as possible. In the case of the Cessna Caravan, that was done with the aid of a tail stinger. The stinger (Fig. 8) is 1.8 m long and the Cs magnetometer sensor is located in the far end of the stinger. The Twin Otter installation includes two Cs-2 magnetometers and the sensors are located in the wingtip pods between the EM coils (see Fig. 6). The vibration of the sen- Magnetometers sor in respect to the aircraft fuselage and wing – even a small vibration – can increase noise that is recorded by the magnetometer. Of course, the vibration produces this response from the magnetic material close to the sensor. Therefore to minimise excessive noise, all screws, bolts and other materials used in the installation were of an absolutely nonmagnetic nature. Sometimes even the pop rivets used in the aircraft were replaced with less magnetic ones. The Cs sensors are far more resistant to the quite strong electromagnetic fields caused by the EM transmitter in the left wingtip of the Twin Otter compared to the proton magnetometer sensors. However, in order to reduce the small interference from the EM transmitters, the Cs sensors were installed inside modified Helmholtz coils. The second step in minimising the influence and disturbance of the aircraft was to apply software corrections to reduce the offsets in the magnetic measurement values caused by the aircraft. Presently, an integral part of the GTK magnetometer system is an active automatic digital compensator (AADCII) made by RMS Instruments. The compensator includes a three-axis fluxgate magnetometer to observe and register the various aircraft orientations. With this compensator the typical improvement ratio is 10–20 for magnetic total field measurements. 33
Geological Survey of Finland, Special Paper 39 H. Hautaniemi, M. Kurimo, J. Multala, H. Leväniemi and J. Vironmäki 34 Fig. 8. Tail boom installation of the Cs magnetometer sensor in the Cessna Caravan. Photo: Kai Nyman The radiometric measurement unit consists of a set of NaI detectors and a gamma-ray spectrometer (Exploranium 1996). Nowadays the most common dimension of detectors used are 10.16 x 10.16 x 40.64 cm (4 x 4 x 16 ins) size prismatic NaI crystals (volume 4.195 litres) that are packed four side by side and a fifth one on the top of the four in a thermally insulated box. So the total crystal volume of one such package is then 21 litres. The four detectors shield the fifth from gamma radiation originating from the ground. The fifth crystal is for measuring atmospheric gamma radiation and is commonly referred to as “upward looking” crystal. In order to increase the detector volume and thus the measurement accuracy, several crystal boxes can be connected to the spectrometer. Today both aircraft are equipped with two crystal boxes. There are several aspects to be considered when choosing the location for the crystal boxes in the aircraft. The mass between the detector and the outside air should be kept to a minimum, and furthermore, it should be constant during the survey flight. In some aircraft, such as the Twin Otter, the fuel tanks are located below the cabin floor. Thus the crystal boxes are installed as far as possible from the tanks. Gamma radiation attenuates when travelling through the full fuel tanks; the attenuating effect is not constant during the flight. As the weight of the crystal boxes is considerable (each crystal box weighs about 104 kg.), their location has some influence on the Gamma-ray measurements aircraft’s centre of mass. Placing these boxes too far from the aircraft’s centre of gravity can compromise the safety of the plane in some extreme situations. In the Cessna Caravan aircraft the fuel tanks are located inside the wing and sufficiently distant from the crystal box, so that their effect is minimal. In order to minimise the background radiation caused by the aircraft itself, all the self-luminous signs and aviation instruments were checked for gamma radiation and changed to less radiating types. Certain types of aviation batteries have been found to have high potassium content, and those have been replaced by less radioactive models. One problem with the airborne spectrometers used to be the drift of the energy stabilisation. The spectrometer (Exploranium GR-820), which is the mainstay of GTK’s operations, monitors continuously every individual crystal and in turn adjusts the spectrum automatically. Besides automatic gain control, modern spectrometers have several software functions to aid in the quality control activities like daily source checks and follow-up of crystal resolutions. Spectrometer data are recorded, summed once every second, which ensures enough pulses for each measurement to avoid noisy data. Still this represents the smallest reasonable sampling area (footprint), because radiation from the ground is scattered over quite a large area when measuring at the altitude of 30 metres.
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Fig. 4. Geomagnetic total field var
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a) b) c) Geological Survey of Finla
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Figures 11a-b. The Finnish anomaly
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To test the method, the details of
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Fig 14a. Percentage of ferrimagneti
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Data versions and corresponding sma
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Aarnisalo, J., Franssila, E., Eeron
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Time dependent reductions for aerom
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Today, we are able to do numerical
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Increasing magnetic permeability ca
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half-space as geological noise. In
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Interpretation of apparent resistiv
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Thin plate model of Hallaperä mine
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We have demonstrated some character
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three most important radioactive is
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Spatial resolution The resolution o
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Released potassium is dissolved in
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Fig. 2. The Central Lapland granite
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The mineral exploration area in the
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the properties. For the purpose of
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The Virtasalmi area had previously,
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physical data sets due to their com
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Grant, F.S. 1985. Aeromagnetics, Ge
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